Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!lll-winken!uunet!mcsun!unido!mpirbn!p576spz From: p576spz@mpirbn.mpifr-bonn.mpg.de (S.Petra Zeidler) Newsgroups: comp.periphs.scsi Subject: Re: Can CD ROMs be read by an eraseable optical disc drive? Summary: You can build almost anything if you really want to, but ... Keywords: MOD physics Message-ID: <1146@mpirbn.mpifr-bonn.mpg.de> Date: 27 Aug 90 20:24:21 GMT References: <6170@hub.ucsb.edu> Reply-To: spz@specklec.mpifr-bonn.mpg.de (S.Petra Zeidler) Distribution: comp Organization: Max-Planck-Institut fuer Radioastronomie, Bonn Lines: 48 In article Chuck.Phillips@FtCollins.NCR.COM (Chuck.Phillips) writes: >Questions: Would it be possible for the same optical device to read both >types of disks? Alternately, would it be possible to physically fit the >required optics for both formats into a single box? A rewritable optical disk (magnetooptical) works this way: -writing: the laser heats a small patch of disk => the magnetic suszeptibility rises (Curie law, sp?) in that patch => a rather weak magnetic field can therefore switch the direction of the magnetic field in that patch (and in that patch only; thus you need no very fine collimated fields). After writing the patch cools and the magnetic field 'freezes' (so don't put a MOD into a hot oven >:-) -reading: a weak polarized laser, that doesn't heat the disk much, is shone onto the magnetized surface => the magnetic field twists the vector of the polarisation (Faraday effect) => the reflected light will pass an analysator (another polarisator) or not, depending which way the vector of the polarization is pointing; a photodiode measures the 'outcome' A CD is read by reflection: a laser shines onto the material and is reflected either this way or that, depending onto pit or no pit, and generates a zero or a one (and as long as you don't melt the disk down, heat or magnetic fields shouldn't harm it :) The only setup you could use both for MOD and CD reading would be the spin drive and the reading laser, but assembling both into one device could stuff the interior a bit (or make the drives larger). But why do it anyway ? It should be easier to do 5 1/4 and 3 1/2 inch floppy reading in one device and noone bothers to do that. Besides, you can use a CD ROM player for music as well; if it was also a MOD drive, it would be a bit clumsy :) (All technical details open to correction; I was only interested in the physics and didn't keep the rest) Greetings, spz --- spz@specklec.mpifr-bonn.mpg.de or spz@mpirbn.uucp or universe!local-cluster!milky-way!orionis-arm!sol!earth!uunet!unido!mpirbn!spz = S.Petra Zeidler | ... in the midst of your laughter and glee, Auf dem Huegel 69 | you will softly and suddenly vanish away, 5300 Bonn 1, FRG | and never be met with again.